Final Production / 4.2d /
Plastics
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Most plastics are produced from petrochemicals. Motivated by the fitness of oil reserves and threat of global warming bioplastics are being developed. These plastics degrade upon exposure to sunlight, water or dampness, bacteria, enzymes, wind erosion and in some cases pest or insect attack but in most cases this does not leads to full breakdown of the plastic. When selecting materials, designers must consider the moral, ethical and environmental implications of their decisions.
Plastics are a broad category of synthetic or semi-synthetic materials made up of long chains of molecules called polymers. These polymers are what give plastics their key characteristic - plasticity. This essentially means they can be molded or shaped when heated or pressured into a wide variety of forms.
Plastics can be synthetic (made entirely from artificial chemicals) or semi-synthetic (derived from natural sources that are then chemically modified). Typically, synthetic plastics are not biodegradable and can take a very long time to break down in the environment. Natural plastics (bioplastics) can degrade much faster under certain conditions. Generally, plastics are lightweight, durable, inexpensive to produce.
Early plastics used from 1600 BCE through to 1900 CE were rubber-based. Prompted by the need for new materials following the First World War, the invention of bakelite and polyethylene in the first half of the 20th century sparked a massive growth of plastic materials and as we identify the need for new materials with particular properties, the development of new plastics continues.
Natural plastics are naturally occurring materials that can be said to be ‘plastic’ because they can be shaped and moulded by heat. Shellac resin for example is produced by lac beetles and has been used for centuries as a sealant and furniture finish. Amber is fossilised tree resin and is often used in jewellery manufacture. Natural rubber latex comes from the sap of certain trees and is used in gloves, tires, and other elastic products.
Semi-synthetic plastics are made from naturally occurring materials that have been modified or changed by mixing other materials with them. Cellulose acetate is a combination of cellulose fibre and acetic acid. It is used to make camera film. Rayon is also derived from cellulose and is known for its soft and breathable properties. It is widely used in clothing, such as shirts and bed sheets. Vulcanized rubber is natural rubber that has been treated with sulfur and heat. This process makes the rubber stronger and more elastic, making it ideal for tires, hoses, and shoe soles.
Synthetic plastics are materials derived from breaking down, or ‘cracking’ carbon-based materials, usually crude oil, coal or gas. This is generally done in petrochemical refineries under heat and pressure. PET, HDPE, Polyethylene, PVC, ABS, Nylon and Teflon are examples of synthetic plastics.
4.2d.1 | Raw materials for plastics
4.2d.1 | Raw materials for plastics
Most plastics are derived from natural materials such as crude oil and natural gas. The starting point of the production process is fractional distillation. This is where the raw material is broken down into ‘fractions’ or into different parts. The heavy fractions give us lubricating oils, and oils used for heating. The lighter fractions give us gas, petrol, paraffin, and naphtha. The chemical building blocks for plastics come mainly from naphtha.
All plastics are based on polymers and they are created by bonding smaller molecules (monomers) together to form chains or polymers. The size and composition of the polymer will determine the properties of the plastic. The start of making plastics involves cracking naphtha into smaller molecules, dependent on their molecular weight. These smaller molecules include ethylene, propylene, butene and other hydrocarbons. The compounds produced are then further refined to produce the base plastic materials.
Naptha cracking facility
4.2d.2 | Structure of thermoplastics
4.2d.2 | Structure of thermoplastics
Thermoplastics are linear chain molecules, sometimes with side bonding of other molecules, but with weak bonds between the chains. The long chain molecules give the plastics their ductility and toughness. The chains do not always lie in straight lines. They can curl up or lie across the overall structure. What makes thermoplastics reformable with heat is the ability of these chains to easily move around each other. This is possible due to the lack of cross-links between the chains. The molecules become weaker during reheating, which allows them to slip over each other. Thermoplastics can be heated, shaped, moulded, cooled, and reheated many times, with little degradation to the material.
Thermoplastics include: Polymethylmethacrylate (PMMA, Acrylic), Polypropylene (PP), Polyethylene (PE), High Impact Polystyrene (HIPS), Acrylonitrile Butadiene Styrene (ABS), Polyethylene Terephthalate (PET), Poly-vinyl Chloride (PVC), Polycarbonate (PC). Each has different properties and uses.
PVC Pipes used for plumbing
Polyethylene (PE)
Most common plastic.
Can be made in different densities.
LDPE - semi-rigid, translucent, very tough, waterproof, low cost.
HDPE - flexible, translucent/waxy, waterproof, good low temp toughness (to -600).
Polypropylene (PP)
Lowest density of all thermoplastics
Polymer resin
Extremely versatile
Stiff
Chemical resistant
Cheap to produce
Poly-vinyl Chloride (PVC)
Can be rigid or flexible when exposed to a plasticizer called Phthalates.
High hardness, flexible when soft,
Good insulator,
Low thermal resistance
High Impact Polystyrene (HIPS)
Versatile material.
Easy to machine, manipulate, shape and construct.
Low cost.
Impact resistant.
Good aesthetics.
Good dimensional stability.
Acrylonitrile Butadiene Styrene (ABS)
Low cost engineering plastic.
Good resistance to impact.
Dimensional stability.
Easy to paint or glue.
Easy to shape, machinability.
Good aesthetics.
Stiff and strong.
Heat resistant.
Polyethylene Terephthalate (PET)
The most common type of thermoplastic polymer resin of the polyester family.
Cheap to produce.
Strong and impact-resistant.
Rigid, or semi-rigid, can hold hot liquids, gases, and alcohol. Thermal resilient.
Polymethylmethacrylate (PMMA, Acrylic)
Excellent transparency
Lightweight and shatter-resistant
Resistant to UV rays and degradation from sunlight
Holds its shape well and resists warping.
Easy to machine
Scratch resistant
Limited chemical resistance
Polylactic Acid (PLA)
Biodegradable
High strength and stiffness
Popular filament for 3D printing due to its low melting point and good layer adhesion.
Limited heat resistance
Brittle
Can be formed using techniques like injection moulding, extrusion, and blow moulding.
4.2d.3 | Structure of thermosetting plastics
4.2d.3 | Structure of thermosetting plastics
Thermoplastics have a long chain structure.
Thermoset plastic cross chain structure. It is these cross links that give Thermosetting plastics their strength and inability to be reformed with heat.
Thermoset plastics are linear chain molecules (the same as thermoplastics) but with strong primary bonds between adjacent polymer chains or cross-links. This gives thermoset plastics a rigid 3D structure.
On first heating, the polymer softens and can be moulded into shape under pressure. However, the heat triggers a chemical reaction in which the molecules become permanently locked together. As a result, the polymer becomes permanently ‘set’ and cannot be softened by reheating like thermoplastics.
Thermoset plastics have several advantages. Unlike thermoplastics, they retain their strength and shape even when heated. This makes thermosetting plastics well-suited to the production of more permanent components as well as larger solid shapes. Additionally, thermoset components have excellent strength, although they are brittle, and will not become weaker in increasing temperatures.
Due to their chemical structure, thermosetting plastics can be difficult to shape in comparison to thermoplastics. Compression moulding is a commonly used manufacturing technique
Polyurethane (PU)
Electrical insulator - resistance.
Good tensile and compressive strength.
Good thermal resistance.
Fairly hard and tough.
Can be easily bonded.
Can be flexible and elastic.
Urea-Formaldehyde
High tensile strength.
Heat resistant.
Low water absorption.
High surface hardness.
Melamine Resin
High electrical resistivity.
Very low thermal conductivity.
High heat resistance.
scratch/stain resistance.
Can be made in a range of thicknesses.
Epoxy resin
Tough.
Chemically and water resistant.
Fatigue and mechanical strength.
Tensile and compressive strength.
Electrical insulation.
Temperature resistant.
4.2d.4 | Recovery and disposal of plastics
4.2d.4 | Recovery and disposal of plastics
As a valuable and finite resource, the optimum route for most plastic items at the ‘end of life’ is to be recycled. Preferably into something that can be recycled again and again. Nearly all types of plastic can be recycled, however, the extent to which they are will depend on technically how difficult it is or whether it is economically viable to do so.
Recycling plastics means turning waste into a new substance or product. This includes composting. Recycling can:
Provide a sustainable source of raw materials for industry.
Significantly reduce the environmental impact of plastic-rich products which give off harmful pollutants in manufacturing and when incinerated.
Minimise the amount of plastic being sent off to landfill sites.
Avoid the consumption of the Earth’s oil stocks.
Consume less energy than producing new, virgin polymers.
Encourage a sustainable lifestyle.
Plastic waste consists of various polymer types. More than 90% of waste is made of thermosoftening polymers, which can be remelted.
To reduce the problems of disposing of plastics they can be designed to be biodegradable. These are Bioplastics and they are plastics derived from renewable sources, such as vegetable fats and oils, corn starch, pea starch and microbiota. Production of oil-based plastics requires the use of fossil fuels and produces greenhouse gases, Bioplastics do not.
Some, but not all bioplastics are designed to biodegrade. Biodegradable plastics can break down in either aerobic or anaerobic environments, depending on how they are manufactured. Bioplastics can be composed of starches, cellulose, biopolymers and a variety of other materials.
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